The present invention relates to gain control circuits and more particularly to an electronic gain control circuit for varying the gain of differential amplifiers.
Of the prior art, the most significant manner presently utilized to gain control differential amplifiers is to shunt a portion of the input signal to ground in response to a control voltage. Although this is a very good method to electronically gain control solid state amplifiers, several disadvantages are related to this method.
The aforementioned gain control circuit requires the rationing of a large diffused resistor to a small diffused resistor. This is most difficult to achieve in typical fabrication processes used today without increasing costs significantly. As the ratio of the resistors is changed due to process tolerances, the amount of variance in gain of the differential amplifier in response to a gain control signal varies, which is undesirous. It is desired that the amount of change in gain per change in control bias be fixed and known.
Another problem arising from this ratioing of resistors occurs with changes in operating temperatures. As the effective resistances of the resistors vary with temperature, the ratio therebetween also varies. Hence, the amount of gain control will also vary with temperature. In a dynamic closed loop feedback system this would normally not be a problem. However, this prior art system cannot be advantageously used in open loop systems.
A further problem associated with the prior art is due to the fact that gain control is accomplished across the relative small resistor of the pair. Thus, the linearity of gain versus control suffers. Additionally, because the magnitude of the control voltage established across the small resistor is quite small, the amount of current split between differential transistor output pairs varies as a function of temperature which is also undesirous.
Thus, there is a need for an electronic gain circuit to overcome the above deficiencies of the prior art.